1. Field of the Invention
The present invention relates to a surface acoustic wave filter having a balance-to-unbalance conversion function.
2. Description of the Related Art
Recently, there has been significant technological progress in reducing the size and weight of cellular telephones. In order to achieve this, in addition to reducing the number and size of components, multi-function components are being increasingly developed. In view of such a background, recent research has been actively conducted on surface acoustic wave (SAW) filters having a balance-to-unbalance conversion function, i.e., a so-called xe2x80x9cbalunxe2x80x9d function, which are for use in, for example, in the RF (radio frequency) stage of cellular telephones. Such SAW filters are used mostly for GSM (global system for mobile communications) compliant devices.
If a balanced line such as a twin lead feeder is directly coupled to an unbalanced line such as a coaxial cable, an unbalanced current flows, thereby, undesirably, causing the feeder itself to operate as an antenna. A circuit called a balun is thus required for preventing the unbalanced current in order to match the balanced line to the unbalanced line.
Many patent applications pertaining to SAW filters having a balance-to-unbalance conversion function have been filed. A well-known SAW filter having a balance-to-unbalance conversion function is illustrated in FIG. 12.
In the SAW filter shown in FIG. 12, a comb-like electrode (also called an interdigital transducer, hereinafter referred to as an xe2x80x9cIDTxe2x80x9d) 11 is disposed on a piezoelectric substrate 10. IDTs 12 and 13 are positioned on the left and right sides of the IDT 101, respectively, in the surface acoustic wave (SAW) propagation direction.
Reflectors 14 and 15 for reflecting a surface acoustic wave in order to improve the conversion efficiency are further arranged in the SAW filter in such a manner that they sandwich the IDTs 11, 12, and 13 therebetween from the left and right. The SAW filter further includes balanced signal terminals 16 and 17, and an unbalanced signal terminal 18.
The SAW filter of this type is called a three-IDT longitudinally coupled resonator SAW filter having a balance-to-unbalance conversion function by performing conversion between the IDTs 11, 12, and 13 using a surface acoustic wave.
In a SAW filter having a balance-to-unbalance conversion function, it is required that the transmission characteristics in the pass band between an unbalanced signal terminal and each balanced signal terminal have amplitude characteristics as equal as possible and 180xc2x0-out-of-phase characteristics as much as possible. The amplitude characteristic and the phase characteristic are referred to as xe2x80x9camplitude balancexe2x80x9d and xe2x80x9cphase balancexe2x80x9d, respectively.
The amplitude balance and the phase balance are defined as follows. If the SAW filter having a balance-to-unbalance conversion function is a three-port device consisting of, for example, a first port corresponding to an unbalanced input terminal and second and third ports corresponding to balanced output terminals, the amplitude balance=A, where A=|20 log(S21)|xe2x88x92|20 log(S31)|, and the phase balance=Bxe2x88x92180, where B=|∠S21xe2x88x92∠S31|. In the above equations, S21 denotes the transfer coefficient from the first port to the second port, S31 denotes the transfer coefficient from the first port to the third port, and ∥ indicates an absolute value. Ideally, the amplitude balance and the phase balance are 0 dB and 0 degree, respectively, in the pass band of a SAW filter.
However, in the related art, SAW filters having a balance-to-unbalance conversion function have had a problem in that balance between balanced signal terminals is low.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a balance-to-unbalance conversion function and having high balance between balanced signal terminals.
According to a preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate and a longitudinally coupled resonator filter including three IDTs arranged along a surface acoustic wave propagation direction provided on the piezoelectric substrate. At least one of the three IDTs has a different phase from the other IDTs. An unbalanced terminal and balanced terminals are provided for the longitudinally coupled resonator filter. Reflectors are arranged to sandwich the three IDTs therebetween, and the reflectors are grounded.
Furthermore, according to another preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate and a plurality of longitudinally coupled resonator filters each including three IDTs arranged along a surface acoustic wave propagation direction on the piezoelectric substrate. At least one of the three IDTs has a different phase from the other IDTs. An unbalanced terminal and balanced terminals are provided for the plurality of longitudinally coupled resonator filters. Reflectors are arranged to sandwich the three IDTs therebetween in each of the longitudinally coupled resonator filters, and each of the reflectors are grounded.
Accordingly, in a SAW filter, a longitudinally coupled resonator filter having three IDTs arranged along the SAW propagation direction is provided with an unbalanced terminal and balanced terminals, thus implementing a balance-to-unbalance conversion function. The SAW filter further includes reflectors arranged so that they sandwich the three IDTs therebetween, thus reducing the insertion loss.
The reflectors are grounded, thereby improving balance between the balanced signal terminals while avoiding reduction of characteristics other than balance, such as a transmission characteristic, when a longitudinally coupled resonator filter having three IDTs is used.
In the above-described SAW filter, at least one surface acoustic wave resonator may be connected at least in series or in parallel to the longitudinally coupled resonator filter, thereby increasing the out-of-passband attenuation to improve filtering characteristics.
Each of the reflectors has preferably 20 to 200 electrode fingers. The reflectors each having less than 20 electrode fingers lead to reduction in reflection efficiency for a surface acoustic wave and high insertion loss. On the other hand, the reflectors each having more than 200 electrode fingers make the SAW filter larger in size.
Therefore, the reflectors each having a range of 20 to 200 electrode fingers make it possible to achieve high balance while avoiding reduction of characteristics in the pass band.
Preferably, the pitch between the IDTs and the reflectors is within a range from about 0.4 xcexR to about 0.6 xcexR, where xcexR represents the wavelength of the reflectors. When the pitch between the IDTs and the reflectors is out of this range, an undesirable ripple occurs in the pass band.
The IDT-reflector pitch ranging from about 0.4 xcexR to about 0.6 xcexR prevents a ripple of an output signal in the pass band and improves a transmission characteristic.
Preferably, in the longitudinally coupled resonator filter, the end IDTs located beside the center IDT have an inverted phase relative to each other; or, in at least one of the plurality of longitudinally coupled resonator filters, the end IDTs positioned beside the center IDT are phase-reversed relative to each other.
The end IDTs positioned beside the center IDT are phase-reversed relative to each other, leading to a more reliable balance-to-unbalance conversion function.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.